It is well known that a reduction in the size of a seal gap between a stationary (i.e., non rotating) seal member and a movable member, such as a rotating shaft surrounded by the stationary seal member, changes dramatically the amount of leakage through the seal gap. For a given seal gap--for example, one of 5 mils (0.005 inches)--a reduction in the size of the seal gap to one-half mil (0.0005 inches), i.e., an order of magnitude reduction of the seal gap, will reduce the leakage of an incompressible fluid through the gap by 10.sup.-3, i.e., a reduction in fluid flow (leakage) to one one-thousandth of the prior flow. Many seal applications serve to separate different media located in adjacent portions of a housing--for example, a lubricating liquid medium on one side of the seal is separated from a gaseous medium on the other side of the seal. These media may be required to be at different pressures--for example, the lubricating medium may be at high pressure and the gaseous medium may be at a lower pressure. Irrespective of the pressure difference between the media, it is desirable to minimize the leakage of one medium into the other medium for certain applications, since, for example, oil leaking into a gaseous medium may well contaminate that medium. Thus for seal applications where there is a controlled leakage across the seal, it is important to maintain the seal gap as small as possible, thereby minimizing the mixing of the two media, and likewise, requiring smaller equipment for handling the in-flow and leakage of liquid into a gas containing region. In certain applications where long lived seals are required, bushing type seals have been used between a rotating shaft and a housing to isolate a lubricating medium, employed, for example, for lubricating shaft bearings, from another medium within the housing employed for other purposes within the device. Bushing seals run at relatively high clearances proportional to the shaft size, such clearances being in the neighborhood of 5 mils (0.005 inches), and therefore have a significant amount of lubricant leakage through the seal. It is desired by manufacturers of certain equipment such as electrical generators, particularly hydrogen cooled machines, pumps, compressors and other turbo machinery, to have a high reliability seal, i.e., a seal with little wear and long life, with as low a leakage therethrough as possible. Thus, if the leakage flow through a bushing seal could be reduced by an order of magnitude, e.g., from 5 mils to 0.5 mils, without sacrificing the long life and reliability of the seal, it would be a boon to such manufacturers.
The problem associated with seals having such small seal clearances is that the seal clearances frequently are dependent upon the speed of rotation of the rotating shaft. This is not ordinarily a problem during normal operation of the device, since many such devices operate at a constant speed. However, problems of seal wear and of maintenance of seal clearance can arise during the periods of transient operation of the equipment employing the seal, i.e., during start-up or shut-down of the rotating equipment. Of course, certain equipment has varying speeds of shaft operation, and the speed of rotation of the shaft influences the seal gap by either increasing or decreasing the seal gap (seal clearance).
In addition, in seal applications with very low clearances and therefore very little leakage, the quantity of cooling fluid transported to the seal member particularly adjacent to the seal gap or clearance is significantly reduced. This may result in undesirable heating of the seal components, causing distortion thereof which will vary the size of the seal gap and reduce seal lifetime.
It is therefore desirable to provide a seal for a rotating shaft designed with a very small seal gap wherein the size of the seal gap is maintained reasonably constant during variations in shaft speed. In addition, it is desirable to provide a seal with a reasonable flow of coolant along most, if not all, of the seal gap in order to prevent thermally induced distortion of the seal components. This invention provides a seal structure which solves the above-described problems.